We used fracture label to examine the partition of the transmembrane proteins that bear CD4 antigens in human lymphocytes. Experiments in both capped and uncapped cells show that, on freeze-fracture, the antigens are preferentially labeled with the inner (protoplasmic) halves of the membrane. This partition illustrates the transmembrane attitude of the antigen molecules and is a probable consequence of interaction of the protein with other components of the membrane or with the cytoskeleton.